While testing out the new 'zombies spawn other zombies' thing, I noticed that zombies were spawning on the slabs I put on the roof of the testing chamber and at the same time I noticed that they didn't burn.
I made a quick test with a walkway and they do burn on normal blocks and in fact run right for the slabs.
So there are two bugs here:
1. Zombies and Skeletons don't burn while on slabs.
2. Zombies (and other mobs? haven't checked this yet) can spawn on slabs.